{ diff(x1, t) = (1*(1 - x4)*(k1*(x2*1/2)^4/(x2*1/2 + k7)^4 + k10)*x3 + (-1)*k2*(x1*1/100)^2/((x1*1/100)^2 + k8^2))/k12,
  diff(x2, t) = (1*k3*(1 - k9/(x1*1/100 + k9)*1/(1 + k11)) + (-1)*k4*x2*1/2)/k12,
  diff(x3, t) = ((-1)*(1 - x4)*(k1*(x2*1/2)^4/(x2*1/2 + k7)^4 + k10)*x3 + 1*k2*(x1*1/100)^2/((x1*1/100)^2 + k8^2))/k13,
  diff(x4, t) = (1*k5*(x1*1/100)^4*(1 - x4) + (-1)*k6)/k12 }